Drum Trap Replacement & Vent Help

Hi all, got myself a 1928 bungalow that I'm slowly working to remodel. I'm currently busy with the attic, but the bathtub drain has given me problems twice in the past year. We've got an old drum trap of course, and the tub starts to drain really slowly after a few months, then I try to clean it out with some Glugg or Drain-O and it ends up clogging completely. Both times I've been able to clear it with a plunger and a lot of effort.

From all the horror stories I've read about the drum trap caps, I don't even want to try opening it (it's in the bathroom floor). I'm thinking of just getting rid of the drum trap and putting in a p-trap. I have full access from the unfinished basement. I'm going to tackle the plumbing later, and don't want to turn this into a big plumbing job (i.e tearing into the main stack, etc), but just want to get it to work properly and then I'll come back and make it clean and fully updated in a couple years.

My concern is needing to add a vent line to the drain line if I take the drum out and put a p-trap in. I'm pretty sure I need this, but would like some help from you guys on this.

Here is a sketch of what it looks like in the basement. You can see the drum inlet and outlet are at 90 degrees to each other and the branch going into the main stack is at an angle (i.e. not perpendicular to where the tub drain comes down). Also, the sink and toilet come into the man stack just above where the tub drain does.

What I would like to do is the below, but I'm thinking this won't work without a vent. Is that correct?

Here's a way I could add a vent. Things are pretty tight with the vent stack, main sewer stack and such, so it will be a bit tough to get access, but it's doable. Also, I'm basically thinking of bringing the tub drain straight down a couple feet to line up with where the tub's wye is in the main stack already. No problems with that, right? You can see there would be a knee in the drain line because of the way the main stack wye is angled. I'm also thinking of using couplers where the arrows point. Again, this would all likely be replaced in a few years and I just don't want to work on getting the old lead joint broken out. If this is simpler than I'm thinking, I'd appreciate some guidance.

Thanks for any and all help. I can upload pics as well, but the space is tight and a bit cluttered, so I think the sketches give an even better view.

Ah, ok, completely missed that. Thanks. I'm learning as I go here. I usually do a lot more research, but I have barely even started thinking about the plumbing on the house. This issue just sort of popped up on me.

I'll have to think more about how I can get a vent in for this. I'm really pretty constrained.

One way might be to go up off the downstream end of the p-trap and go into the bathroom wall and go around and then connect with the vent line as it runs up the bathroom wall. That would tear up the walls quite a bit and the bathroom itself is in decent condition and I'd hate to tear it up now. It also might still be lathe and plaster on some walls. Some look to have been re-done with drywall, but I don't think all were.

I don't think I have any space to go straight up along the wall near where the p-trap would be, but I might. If I did that, I have a separate vent line from the kitchen that already exits the roof and I could tie into that in the attic.

What are the chances that the downstream end of the drum trap has one end go down and one end go up at the tee, and goes up through the floor, along the wall, and then connects to the main vent stack? Did they ever connect these old drum traps to a vent line?

Also, would an AAV be an option? I wouldn't consider that a long term fix, but could do that for now and then route a proper vent line later.

The previous owner just had new fixtures put in for the tub and had nearly the entire tub fixture wall torn out and re-finished. Wish he would have just had the plumber add a vent line to the tub at that point.